An electrostatic painting apparatus includes a high voltage generating device that applies a high voltage to paint, a paint gun, and so forth. In electrostatic painting performed by the electrostatic painting apparatus, the high voltage generating device applies a negative high voltage to charge paint to a negative potential, and the potential of a paint target object is set to “0” (by grounding the paint target object). Thus, an electrostatic field is formed between the paint and the paint target object to perform electrostatic painting on the paint target object.
The high voltage generating device includes a voltage generating section that generates an operating voltage, and a voltage boosting section (a so-called cascade) that boosts the operating voltage which is generated by the voltage generating section. These sections are typically electrically connected through a low-voltage cable. In the thus configured high voltage generating device, a wire breakage may occur in the low-voltage cable. Thus, it is desired to develop a technique for accurately detecting such a wire breakage.
For example, there are disclosed techniques for detecting an abnormality that occurs in a high voltage generating device. Japanese Patent Application Publication No. 6-320066 (JP-A-6-320066) discloses a technique in which a “di/dt value” is defined as the maximum value of an increase in output current of a high voltage generating device allowed while an output voltage of the high voltage generating device increases by a predetermined value, and the “di/dt value” is evaluated every predetermined time. Specifically, an overload state (high-voltage abnormality) is detected in accordance with the amount of variation over time in current value during application of a high voltage. For example, a high-voltage abnormality is determined in the case where the current value increases by 40 μA or more in 10 msec.
Japanese Patent Application Publication No. 2005-66410 (JP-A-2005-66410) discloses a technique in which an absolute sensitivity circuit that stops the supply of a high voltage from a high voltage generating device when an output current from the high voltage generating device exceeds a prescribed maximum value is provided, and a slope sensitivity circuit that stops the supply of a high voltage from the high voltage generating device when the amount of increase in output current from the high voltage generating device per unit time exceeds a prescribed maximum increase amount is provided.
According to such techniques, the following advantages are obtained. (1) In the case where the paint gun approaches the paint target object at a low speed, a high-voltage abnormality can be detected if the output voltage exceeds the maximum set value. (2) In the case where the paint gun approaches the paint target object at a high speed, a high-voltage abnormality can be detected if the amount of increase in output current per unit time exceeds the maximum set value (even if the output voltage does not exceed the maximum set value). (3) A high-voltage abnormality can be detected if a leak current increases due to a paint stain or the like on the paint gun or the like to cause the output voltage to exceed the maximum set value.
In the case where a wire breakage occurs in the low-voltage cable of the high voltage generating device, a high voltage cannot be output. Therefore, the presence or absence of a wire breakage can be detected by monitoring the current value and the amount of variation over time in current value using the techniques disclosed in JP-A-6-320066 and JP-A-2005-66410.
In the initial stage of a wire breakage in the low-voltage cable, however, it is often the case that the wire is not completely broken and the broken portion becomes continuous again as the electrostatic painting apparatus operates. It is difficult to reliably detect such a momentary wire breakage (a so-called chattering) in accordance with the current value and the amount of variation over time in current value, because apparently a normal continuous state is immediately restored. That is, it is difficult to accurately detect a momentary wire breakage in the low-voltage cable using the techniques disclosed in JP-A-6-320066 and JP-A-2005-66410.
If such a momentary wire breakage can be reliably detected, effective preventive measures can be taken such as by replacing the low-voltage cable before the low-voltage cable is completely broken. Thus, the development of a technique that allows accurate detection of a momentary wire breakage in the low-voltage cable is desired.